Semiconductor memory manufacturers use spare memory cells to repair defective memory cells and improve wafer yield. Spare memory cells are commonly called redundant memory cells. Thus, redundancy refers to schemes utilizing spare memory cells. Content addressable memories (CAMs) are a popular means of implementing redundancy. A CAM stores the address of an electrically defective memory element. The defective memory element is replaced with a redundant memory element when the CAM is activated.
Placement of CAMs on a die relative to a memory array varies. In one prior UV erasable electrically programmable read only memories (EPROMs) the CAMs are located to the side of the memory array. The CAMs may then be addressed by memory array row selects, thereby decreasing circuitry associated with CAMs. To prevent the CAMs from being erased along with the memory array, the CAMS are sealed to prevent UV light from reaching and erasing them. As a result, CAMs in UV EEPROMs are quite large. This is a disadvantage as large CAMs increase die size and decrease wafer yield.
In one prior electrically erasable programmable read only memory (EEPROM) CAMs are placed below the main memory array in separate mini-arrays. Due to their location the CAMs cannot be driven by main array wordline selects. The additional circuitry required to drive the CAMs increases die size.
The guard rings surrounding each CAM mini-array also increase the die size of the prior EEPROM. Guard rings surround the CAM mini-arrays to prevent electrons injected into the mini-arrays from straying into other circuits. Guard rings also attract certain impurities into the guard ring structure and away from the arrays which guard rings surround. Guard rings increase die size in disproportion to their actual size because lay-out design rules dictate that circuitry must be spaced apart from guard rings.